Strong interactions among different degrees of freedom in solids can lead to exotic phases of matter, often characterized by complex order parameters and emergent collective excitations. While conventional techniques such as scattering and transport measurements are effective at probing the amplitude of these excitations, they typically lack sensitivity to their phase components (Coleman, Introduction to Many-Body Physics, 2015. https://doi.org/10.1017/cbo9781139020916 ). This limitation poses a challenge for studying ground states with phase modulations and interactions that specifically couple to the phase of collective modes (Hamidian et al., Nature 532(7599), 2016). In this chapter, we employ phase-resolved coherent phonon spectroscopy (CPS) to uncover a previously hidden spin-lattice coupling in the van der Waals antiferromagnet FePS \(_3\) , a coupling that has remained undetected using phase-insensitive methods such as Raman and X-ray scattering. By combining experimental observations with analytical modeling, we show that the magnetic order in FePS \(_3\) couples selectively to trigonal lattice distortions via the partially filled t \(_{\text{2g}}\) orbitals. This magnetoelastic coupling is linear in both the magnetic order and the lattice distortion, rendering it invisible to conventional inelastic scattering techniques. Our results not only reveal a subtle yet significant interaction in FePS \(_3\) , but also highlight phase-resolved CPS as a powerful and sensitive method for probing hidden coupling mechanisms in quantum materials.